Tolerance and acclimation to zinc of field-collected Daphnia magna populations.

The zinc tolerance of two Daphnia magna populations collected at a zinc contaminated site was studied. One clone was isolated from each population in order to determine interclonal variation in zinc tolerance. 48hEC50-values, life table parameters, carapace lengths and cellular energy allocation (CEA) were used as test endpoints and compared with the results obtained with a standard laboratory clone. The natural clones were more tolerant to acute zinc toxicity (up to a factor of 4) and exhibited a higher reproduction rate (factor 2) and carapace length (factor 1.2). The optimal zinc concentrations for the natural clones ranged from 80 to 200 microg Zn/l. When cultured without zinc, the natural clones gradually lost their zinc tolerance. Therefore, the environmental relevance of using toxicity data obtained with organisms (natural, as well as laboratory clones) acclimated to culture media containing no or very small amounts of zinc can be questioned.